Heretofore, a seat has been known that is equipped with a seat cushion and a seat back and is capable of adjusting a seated posture of a seated person. For example, in the car seat recited in Patent Reference 1, a seating portion of the car seat includes a seating portion reinforcement frame that supports a suspension grid, and an oscillation unit that generates oscillatory movements in the seating portion. This produces an alternating upward massage effect at the lower side of the thighs and small movements of the lower portion of the spine, at the same time as moving the pelvis rearward to a correct position. Further, a seat recited in Patent Reference 2 is equipped with a lumbar support device that supports the lumbar region of a seat occupant, a pelvis support device that supports the pelvis region, a detection unit that detects turning of the pelvis, and a control unit, and maintains an optimum seated posture.    Patent Reference 1: Japanese Patent Application Laid-Open (JP-A) No. 2001-353039    Patent Reference 2: Japanese Utility Model Application Laid-Open (JP-U) No. H3-19344